My triathlon team swims a couple times a week at a nearby lake. Its an open invite for the team but we are pretty careful about their skill level to swim at least 500 yards straight. Plus most new people will use a wetsuit for buoyancy. We've moved location a few times to be as accommodating as possible to new open water (OW) swimmers so the route has as many bug outs as possible to shore and no motorized boats.

As there are many people coming into triathlon or need experience with OW, every other weekend we run a much larger group and put as many kayaks in the water as possible to help out. They have to wait for that kayak support weekend if they are nervous but many of us are training for our own races and can't waste our training time by being personal sitters for new swimmers. We have to get our distance in too.

Its about as safe as you can get in OW minus personal responsibility.